CALIS

CALIS-1 and CALIS-2 are pulse sequences for the accurate calibration of the RF field strength for an indirectly observed spin are introduced. CALIS-2 is intended for calibration of e.g., ¹³C or ¹⁵N pulses on natural abundance samples whilst CALIS-1 is recommended primarily for enriched samples. Both experiments can be performed without prior knowledge or guess of the RF field strength and no delays in the pulse sequences are critically dependent on coupling constants.

Sequence

For = /2 the signal goes through a zero crossing. The same phasing is applied to all of the spectra recorded with the different vales of . The delay = 2(¹J_IS)^-1 and the delay '= -5 . The delay = ₀-5 /2

The gradients are set so as to fulfil either of the equations:

1) _IG₁ - (_I + _S)G₂ - (_I - _S)G₃ = 0

2) _IG₁ - (_I - _S)G₂ - (_I + _S)G₃ = 0

For I and S being ¹H and ¹³C respectively the first equation is fulfilled for G₁=+5, G₂=-2 and G₃=+10. The gradient G_p is optional and can be of any size.

The recommended phase cycle is an even number of steps out of:

• ₁ = {x, -x, -x, x},
• ₂ = {x, x, 4(-x), x, x},
• ₃ = {4(x), 4(y), 4(-x), 4(-y)}
• Receiver phase {x, -x}.

Results

a) shows an example calibration of the ¹³C RF field strength using the CALIS-2 sequence on mannose and shows both the and anomeric signals. b) as for (a) but using the CALIS-1 sequence. The clean zero crossing illustrates how effectively any residual signals from protons attached to ¹²C are suppressed.

Reference

1. Andrew J. Benie, Ole W. Sørensen New methods for calibration of the RF field strength for indirectly observed nuclei J. Magn. Reson. 180 317-320 2006

Code and procedures for Varian and Bruker.